Signal-seeking tuner



March 7, 1950 E. G. HILLS SIGNAL-SEEKING TUNER Filed Oct. 15, 1945 amr' INVENTOR. El??? G. HILLS Mam $4 ATTORNEYS Mi E w u (IT 2 g 5 M 9m v 2 at -IIIJ MM A 1 r.o A w l O 6 o 6 $2 52 $2 58: E n z 0 $8 38 y. m. E o. a mv l Patented Mar. 7, 1950 SIGNAL-SEEKING TUNER Elmer G. Hills, Chicago, 111., assignor to Belmont Radio Corporation, a corporation of Illinois Application October 15, 1945, Serial No. 622,465

14 Claims. 1

This invention relates to signal-seeking tuners and more particularly to such tuners for radio broadcast receivers of the type which sweeps over the tuning range until a station is reached from which the signal is above a predetermined minimum acceptable signal strength and which rests in such position.

In certain broadcast receivers it is desirable to procure the ease of tuning characteris ic of pushbutton tuning apparatus while at the same time it is desired to avoid the presetting feature characteristic of such tuners. For example, in the case of automobile receivers the tuning of the local stations of greatest signal strength varies from place to place so that it becomes impracticable to set up a push-button tuner in such a way as to be useful as a car is driven over considerable distances.

It is an object of the invention, therefore, to provide a new and improved tuner for a radio broadcast receiver which has the ease of tuning of conventional push-button tuners but does not require any prese ting operations.

It is another object of the invention to provide a new and improved tuner for radio broadcast receive s of the signal-seekin type; that is a tun r in which one operation of a push-button will cause the tuner to seek a s ation transmitting signal of acceptable field strength and to hold on to such stations until the pu h-button is operated a second time, when the cycle is repeated.

In accordance with the invention. an automatic signal-seeking tuner for a radio receiver comprises an ediustable tun ng a aratus for t e rece ver,

power means for actuating the tuning apparatus, I

and a normally unbalanced electrical network for controlling the power means. The tuner also includes manually operable means for unbalancing the network in an opposite sense to initiate operation of the power means. and means responsive to the reception of a signal by the receiver for restoring the network to its initial sense of unbalance to arrest actuation of the tuning apparatus.

Referring now to the drawing, Fig. 1 is a circuit diagram, partially schematic, of a broadcast radio receiver including a signal-seeking tuner embodying the invention; Fig. 2 is a graph representing an operating characteristic of the frequency discriminator of the system of Fig. 1; while Fig. 3

is a circuit diagram of a modified form of signal seeking tunercontrol circuit.

Referring now to Fig. l of the drawing, there is represented a complete radio-broadcast receiver including an automatic signal-seeking tuner em- 2 bodying the invention. This receiver comprise a radio-frequency amplifier unit Ill having input terminals connected to an antenna-ground circuit II and output terminals connected to an oscillator-modulator unit I2.

This latter unit is in turn connected to an intermediate-frequency amplifier I3, a frequency discriminator I4, described in more detail hereinafter, a portion of which serves also as a modulation signal detector. The detected modulation signal is applied by way of a circuit I5 and a coupling condenser I6 to a volume control resistor I! having an adjustable contact Ila which, in turn, is connected through an audio-frequency amplifier I8 to a signal rep-roducer such as a loud speaker I9 having a field winding I9a constituting a portion of the power supply filter. The radiofrequency amplifier I0 is provided with an adjust able tuning unit schematically represented at Illa. while the oscillator-modulator unit is provided with an adjustable tuning unit I2a. An automatic amplication control or A. V. C. bias is developed in any suitable manner as by means of an A. V. C. diode rectifier 55a coupled to the discriminator load circuit through coupling condenser I6 and provided with a load resistor I517. The A. V. C. bias so developed is applied, as indicated, to one or more of the vacuum tubes of the units I0, I2 and I3 to maintain the amplitude of the signal input to the frequency discriminator I4 approximately constant for a wide range of received signal intensities.

The radio receiver described, per se, forms no part of the present invention and operates in accordance with conventional principles so that a detailed description of its operation is deemed unnecessary. In brief, however, signals intercepted by the antenna system I I are selected and amplifled in the unit I0, converted in the unit I2 into intermediate frequency signals, further selected and amplified in the unit I3, and detected in the unit I4 to derive the modulation signal. This signal is impressed upon the audio-frequency amplifier I8 wherein it is further amplified and applied to the loud speaker I9 for reproduction. The A. V. C. connection from the unit I4 serves to maintain the average signal input to the unit I4 approximately constant and thereby avoids overload of the receiver for strong signals, while providing maximum amplification for weak signals.

' The automatic signal-seeking tuner of the invention, as="embodied in the receiver described, 'comprises the adjustable tuning apparatus or units I01: and I2a and power means, such as a 3 reversible electric motor 20, connected by a shaft or mechanism indicated schematically at 25 to actuate the tuning apparatus Illa, I2a. The tuner also includes adjustable vacuum tube impedance means for controlling the energization of the motor 20. This impedance means may comprise a pair of adjustable-impedance space-current paths 22a, 22b of a twin-triode vacuum tube 22 connected selectively to control the'energization of the motor 20 for operation in either direction. As illustrated, the motor 28 is of the two-phase induction type provided with one phase-winding- 20a connected directlyacross alternating-scum rent supply terminals 23, which may be the usual house wiring circuit. The-motorxZfl is provided? also with a second phase winding energized with one polarity from the-supply terminals 23.,

through a unity-ratio transformer 24, a phase shifting condenser 25, and an adjustable imped-i ance transformer 26, or with the other polarity directly from :thesupply terminals 123* through theephase shifting condenserifi and an. adjustable-impedance transformer 21. The secondary windings .of the transformer 26 and 21 are adapted to be selectively short circuited through the space-current pathsZZa and ;22b,;respectively, otrthaadjustable-impedancevacuum tube 22 Thezsignal-seeking tuner of the inventionnalso includes a normally unbalanced electricalmetworktfor developing a control signal: :tocontrol then impedance means comprising the; vacuum tubew22 and-transformer 26 and '21,:thereby to control :the electricmotor. 2%. This network in:- cludesa paircof-space-currentpaths 28a and-.,28;b

otla vacuum tube 28 connectedin parallel: across aesuitablesource +B byway =-of conductor 29 andianode load resistors 30 and 3t respectively; Thercathode of the vacuum :tube 28 ,is connected toithe junction of avoltagezdividercircuit come prisingresistors 32 and l 33 .con-necteddn series across the source: +13.- Itwill-be-understood that the adjustable-impedance :tubes- :22 and -28 may be either of the highevacuumtype having 3'001'1': tinuous bias-impedance characteristic or .of i .the gaseoussdischarge type having-pa discontinuous bias-impedance characteristic.

Thea-network also includes couplings; between thewspace :current paths 28a and 28b effective to cause: either path rendered conductive-toassume all ota-the network current. To this. end:there areprovided a pair of voltage dividers-individw ually connected between the anodes ofth'e, space: current, paths, 28a, 28b and ground,.; comprising -in:-series: resistors 34, 35,".andr3'6; respectively. The grid of the path 28a is connected .to thegjunca .tiomof resistors 34 and 35, while: the gridiof. path 2812His connected to the junction of resistors-36 andj'l. In. order to cause agivenione of "the paths :28a,-28'binitia1ly to assumethe entire: net'- workiicurrent upon 1 energizatiomof the network; :one 10f the-load resistors 3d, is-made substan, .tially reater than:the-other; for example, the resistor sit-may have a-.va1ue- .twiceuthat of-the resistor 3 l.

The; J -network: also includes -a voltage-divider circuitrcomprising resistors Stand 39 connected 'inzseries across the source t-l-B through. the hem winding l fiaaof the sound reproducerl9 and thus excited at, a potential lower than +B;l The .-:output signal of, the network is derived-between the terminal 40 at the junction of resistor 3i'l ;and theaanode of space-current path28a andthe terminalsfll at the-junction of resistorsSt ,andx39.

-'Iheacircuitv constants. are so .selectedzlthat" the networklisgeifective tOLdQ-VBIOQ, a control signal between the terminals 49 and 4| only when the other path 281) assumes the network current, that is, only when the network is unbalanced in a given sense. The control signal between the terminals 49 and t! is applied through a reversing switch 42 and resistors 43 and M to the control grids of the paths 22a and 22b respectively, of the adjustable-impedance vacuum tube 22 for controllillggthe impedance of one or the other 'ofthe impedance paths 22a, 22b in accordance with the sense of the unbalance of the network and the position of reversing switch 42.

The rSlgl'lEtl-Sflkillg tuner also includes manually operable means for unbalancing the netiwork described inopposite sense to modify the control-signaldeveloped at the terminals 40, 4|, thereby totinitiate.operation of the motor means in a direction dependent upon the position of switch 42*. This manually operable means may comprise a starting push button or a conventionat multiestation. pus-hebutton:tuner- Orr as illustrated; a combination. of; theseetwo :devices: Specifically, thiswniea-ns comprises :a tweeposition push-button switch- 45 connected to impress the SOIIFCBr-I-BrllPOIlthG: control grid of the path 28b through an isolatinggrid resistorlfir The -two-; position switch 35 is mechanically; connected by a 'shaft or mechanism indicated;schematically at; ll to. theireversingswitch :32 so-that; the-two switches are connected for operation in unison: In 2 parallelwith; the: contacts of the. :push -,button switch 45*;are-theseveraicontaets of aamultir. station selector-48i- OfrthfifiOIIlHllltQtOI type come prising a series of commutatondiscs 41%;;4819; 33a and r4801 :and cooperating .;pushebutt'on contacts 49a, {39-21, lilo and:fifidmespectively; The station selectors istrepresented schematically since ,it forms no part of the present invention.

The "signal-seeking tuner also includes means responsive to 'the'reception ofra signal by the --receiver:v for rebalancing; the network to arrest,-;:ac.- tuationr of the tuning, apparatus! by .themotor 20. To this endgganrzamplifiedi:audio-fnequency signali developed-at the ianodersof the amplifier I 8 is applied .throug-h a 1 coupling-condenser '59 to the control grid 'of the space-current path 282) to-renderthis. path non-conductive; whereby the entire network current is. assumed by the space-, current path 28a.v

The signal seeking; tuner: also includes Eddie tional means: responsiveto j the .-rec,eptionl:of -:a signal.-:fo r applying an auxiliary-control signal to. .;-the-l-motor 20.: by way I of the -,;adjustableeiml-. pedance vacuum. tube 22' until the receiver is accurately tuned to; thearecei-ved signal; The means. fordeveloping this}:auxiliary-control Sig; nalisthe frequency discriminator MI This dis,- criminator may be-rofnconyenti-onalform com;- prising a primary tuned circuit -:I4a-,-,inductively coupled to a secondary tuned-circuit,- Mb andzprovided with 'a circuit for coupling-the signali across the. primary circuit E 4a into-n the.--=electrica1amid; point ofthe circuit i 4b.- Thessumand difference of the primary and secondary signals are applied to the discriminator-rectifiers i idaandrlee-of a twin-diode rectifier tuber! 4 2, Thewcathodes "of theapathsi ldand :l liesof; tube My; are connected by .way of -regulating resistors 5 t iand52 etc the load-resistors 431 and 44,-respectively,- and; to ,the, controls-electrodes of the space-currentpaths22a and- 12219, respectively, Carrier-frequency and audio-frequency by-pass' condensers-areincluded inrthecload: circuityof the discriminator; M 31341?- quired while the cathode-'ofvthe vacuumzztube. 22; :'1S:by:p 5@,d" to ground :for warrior-frequency and audio-frequency signals by way of a condenser 53. As is well understood, the frequency discriminator I4 is effective to develop a'n'output signal or auxiliary control signal of opposite polarities for mistuning of the receiverfin'opposite senses.

The signal-seeking tuner also includesmeans responsive to adjustment of the tuning apparatus to either of its extreme positions for reversing the polarity of the output of the network at' the terminals 40, 4| to reverse the directionjof operation of the motor means. This comprises the mechanical connection 2| between the-motor 20 and the reversing switch 42 whichis so arranged that it operates the reversing switch in opposite directions at the two limits of jtravel of the tuning apparatus, as driven by 'the'inotor 20. 1

Referring now to the operation of the signalseeking tuner described, it will be assumed that initially the receiver is not tuned to astation. Due to the fact that the value of resistor 30 is substantially twice that of resistor 3|,when the set is first turned on, the anode potential of the space path 28b of the tube 28 rises more rapidly than that of the path 28a while the coupling from the anode of path 28b to the grid of path 28a renders the latter more conductive. The couplings between the two paths are regenerative so that path 28a assumes the entire network current and the network is normally unbalanced in a predetermined sense. It will be noted that the network output terminal 40 is at a junction of the voltage divider comprising resistor 30, the space-current path 28a and resistor 33 connected across the source +B while the output terminal 4| comprises a junction of the voltage divider circuit comprising the speaker field l9a, resistor 38 and resistor 39. The circuit constants are so selected that, under the conditions assumed, no difference of potential existsbetween the terminals 40 and 4| and, since no signal is being received, no control signal is developed by the discriminator I4 and no control signal is impressed upon the control gridof either of the impedance paths 22a or 22b of the vacuum tube 22. The tube 22 is selected of a type such that, with no signal on its control grids, the impedances of the paths 22a and 22b are low and equal. However, the cathode of tube 22 is biased by the potential difference between +3 and point 4| so that both paths are biased to cut-off in the absence of a control signal on eithergrid. Thus the secondary windings of the impedance transformers 26 and 21 are substantially at open circuit and the excitation of the winding 20b of the motor 20 is zero due to the small but-opposite excitation currents supplied through the transformers 26 and 21. Therefore, the signal-seeking tuner is inactive.

If now it is desired to initiate operation of the tuner to seek a signal from a broadcasting station, the switch 45 is manually operated to one of its positions, thereby impressing'the source +3 on the control grid of the path 28b of tube 28 through the isolating resistor 45.. This positive potential pulse causes the path 28b momentarily to become more conductive than path 28a, whereby it tends to assume current from the path 28a. As the path 28a becomes less conductive, its anode potential rises, still further increasing the potential on the grid of the path 28b 'a'nd the transfer of current continues rapidly ;,until the path 281) assumes the entire network current and the network is unbalanced in the opposite sense,

With substantially no current flowing in the path 28a, the output terminal 40 connected to the anode of the path 28 rises to substantially +B so that a control signal is developed between the terminals 40 and 4| which is impressed through the reversing switch 42 and isolating resistors 43 and 44 on the control grids of the paths 22a and 22b. The polarity of this control signal, as applied to the control electrodes of the paths 22a and 22b, depends of course upon the position of the reversing switch 42. It may be assumed that the push button switch 45 was initially operated in such a direction as to close the reversing switch 42 on its right-hand contacts thereby connecting the output terminal 40 to the grid of the path 22a and the terminal 4| to the grid of the path 22b. The result of this connection is to increase the conductivity and to decrease the impedance of the path 22a and simultaneously to increase the impedance of the path 221). As a result, the transformer 26 is substantially shortcircuited, permitting the energization of the Winding 201) through the transformer 24 to produce rotation of the motor in a given direction. The motor 20 is connected by way of the mechanism 2| to the tuning units Na and |2a and continuously adjusts the tuning of the receiver over its tuning range.

When the receiver is tuned to a station from which a signal of acceptable strength is being received, two effects occur. In the first place, an amplified detected audio-frequency signal from the anode of the audio-frequency amplifier |8 is impressed through the coupling condenser 50 upon the grid of the path 28b of the tube 28. Since this path is already fully conductive, the positive half-cycles of this signal have no efiect but the negative half-cycles normally reduce its conductivity substantially and effect a transfer of the network current from the path 281) back to the path 28a in the manner described above, thus restoring the network to its initial sense of unbalance and reducing to zero the control signal developed between the terminals 40 and 4|. In case the receiver is tuned to a station that is transmitting momentarily an unmodulated carrier signal, the carrier pulse translated by the receiver when tuning to the station will itself trigger the network to produce a similar result. Either of these effects alone tends to stop the motor 20 as soon as the receiver is tuned to a signal of appreciable strength and in certain applications may be an adequate control.

However, for more accurate control of the tuning motor 20, the use of the discriminator I4 is desirable. As is well understood in the art, this type of discriminator is efiective to develop a control signal of opposite polarities for mistuning of the receiver in opposite senses, the control signal characteristic being represented by Fig. 2 which is typical for such a unit. The control signal developed by the discriminator H, as represented by Fig. 2, is applied by way of the isolating resistors 5| and 52 to the control grids of the paths 22a and 22b respectively. It will be obvious from the foregoing description that one or the other of the paths 22a, 22b will be rendered conductive by the auxiliary control signal, depending upon the sense of mistuning of the receiver at the time the control signal from the normally unbalanced network disappears, as described above. By a proper connection of the discriminator M to the control grids of the tube 22, the motor 20 may be driven under the influence of the control signal in a direction to reen ages;

dlice the ini'stuni'ngrofi the receiver?! Bil-designing the discriminator i 4 sand the control circuits of the motorfimwith sufficient;Sensitivitygthe motor ZIImaytbemadeiefiecti-Ve to :tune the receiver very precisely :to-zthef incoming signal. Thus, it .is seeni-thatetheadjustable:impedance tube 22 and impedance;transformers; 25a and 21 comprise means: responsive .joint1y to'= the magnitudes and the polarities ofathecontrol signal developed be-; tweenxterminalsrfi and M andthe auxiliaryjcontrolisignal developediby the discriminator 'i 4 and that: this *meansisis ;;effective to arrest 1 actuation ofithe tuning apparatussonly Whenztheauxiliary controlisignal :from :the "discriminator i 4 is ;re.-. ducedtto' zero.

ItlhasJbeenz found thatiwith the arrangement described t'therecisrrno :de'ad :.'spot at "or near :the center .ofithe. characteristic oi the discriminator l4 COKIESPODdiHgitOfB, precise-tuning;of'the-re-; ceiver-.:.=. Thisuis due-to the'fact thattheaudiofreduency signal: appliedi tozthe unbalanced nete WOTlfaflOIIlz? the" audio-frequency: amplifier l8 regularly triggers the path z8bi'both positively and negativelyso that the. networkrcurrent repeatedlyttansfers back andsforth between the paths28a and 28b, correspondingly 'stoppingiand starting the 'motor-20'under thexcontrol oi the auxiliary signal from the discriminator" M: How ever, the angular. movement of the tuning "apparatus is so small that. its effect 'on the tuning of the receiver: is inappreciable. The efiect. issimilarito that'of'manually jarringlor shock-exciting a mechanical movement' to veliminate its dead spot due to static friction.

If,"after the push-button:switch 45 is operated, nostation is broadcasting within the tuning range of the tuning-units Ifla, 12a, the motor 29 actuates these tuning units to the limits of their ftuning range, whereuponthrough the-mechanismfll it actu'ates the reversing switch 4 2' to make :the other path 221; of the tubei? conductive, therebyefiectivelyto short-circuit the secondary winding of the impedance transformer 21 to-excite the winding 29b with reverse' pola'rity and reverse the direction of 'rotation'of the moton' The motor 20 then tunes-the receiver in the reverse sense, and; if nosignal is still received; it adjusts the'tuning apparatus tdits other extreme of the "tuning range, whereupon the reversing switch dz is again actuated and the cycle" described is'repeated until the receiver is turned on: In ca se the-'receiveris turned 01f while it is"tuned to and receiving a given signal, when it :is- -'subsequently' turned :on small remain tuned'tothisstation'under the. influence of "the :fr'equency"discriminator": l 4, 1 since the unbalanced network always "warms up with the network current being carried by 'thepath 28a,"- under which conditions no' control signal is developed at the terminals 4!], 4l

It-will beunde'rstoodth'at if, inlieu oioperat-- I ing 'the automatic push-button: switch 45," any 'of the "stati'ompush-butt'ons' "Ma-Jed," inc. are operate'chthese 'pushl-buttons act in conjunction with thesele'cton'discs wet- 18d, incl. respectively to operate the tuningmot'or- ZiJ' to a position corresponding to the setting of 1 the par'ticu lar' 'selector disc: In this arrangement +B is maintained continuously'on-the grid of the-path 28b to maintainthenetwork current'through this path-*inspite of audio-frequency signals which maybe impressed thereon from the audio-ire "quency amplifier l8 prio'r-to reaching the station 'fo'r 'whichthe particular selector disc-is set.'*-"

f Th'us, there; is provided 'by 'the automatic sigjna l-seeking tuner 'described. an-*arrangement for automaticallyrtuning la; broadcast-1 receiver ccritinuouslywver its :t-uningra'nge :until itzis tuned to :a'stationof-acceptabl'e signal: strength; whereupon the 'tuningpperation-is arrested. Further, operationaof vthe push-button switch. 45 is :effective to cause the tuner to tune the receiver to successive:stationsgitonly beingunecessary in such an-operationto hold the pushebuttontswitch 45i'closed during-the @period of time required to tune; awaypfromrthe received :signal sufiiciently that theavauxiliar-y -control signal developed by the :discriminatorfl hwill -not-,--..upon the release oi the switch: 45, ,causejthemotor. :ZB-to return the tuning apparatus a to =';the esam'e station: Alternativelyybyrpushing anyone of -the selected station selectors 49a-a-49d, inc the re'ceiver is tuned inz'a conventional zmanneruto ;any ;particular station selected and the descriminator [4 acts to tuneithe receiverprecisely-to such station.

In Fig.-3,-.:there is -represented a modified form ofzithez signali-s'seekingsgtunercontrol apparatus adaptedifor-operationby 'a direct-current motor $6 in "place: of-"the polyphase alternatingecurrent motor 29 of the-system ofFig; l. The-several cir- 1 cuitelements :ofathei system of Fig-"3 correspond;

receiver: '(not shown) and::the 1 reversing :iswitch.

4 zlas ithe systemr'of Fig.1 1.4;.

, The normally :unba-lancednetworkof the system of; Fig: 3 is similar'to that'of Fig.1 with the exceptions-that the :output'terminal 4 I is :con-

divider :resistor 162 -':connected across the source +31% Inthis instance-the transfer ofthe network currentv from the path? 28min which it'isiinitiated tothe pathZBb-is efiectedzby short-circuiting the resistor 30 bymeanssof. contacts 6.3a of a manually. operable. Epushebutto'n :switchr 63.: The network outputterminalss-M 'and24 I: are'xconnected to the reversingewitchulz through iisolatingrresistors 6' 4.

Iniorder to avoid-thea-necessity of holding the pushi-button switch B3-.closedcduringthe interval when; the motor ,iiS tuning. the receiver away frjomxany station :being; received; the; tuner:"in-. eludes means; ion-temporarily reducing: the I gain ofthe receiver simultaneously withthe unbalance ing of the network in order to reducethe efiect of anyssignal then? being "received: on thexoperation'of thernotorsfi 9. Totthisend; the push-button switch 63' :is provided with auxiliary contacts 63h Whichrare' efiective' to connect thesource B to theAJrVsTC. circuitrthrough a time-constant network comprising-a seriesresistor 65 and shunt condenser- 663. Further-, time-constant circuits comprising atparalll-connected:resistor '61 and condenser-Manda parallel-connected resistor 69 and condenser 10"are in'cluded'in the connections from the.discriminatoraii 4 touthe rcontrol grids of the adjustableimpedance-"paths 22'a and 221), r'espectivelyfiin orderttovcontrol =the -speed of the motor during the==t1iiiing operation,-- tie-described The operation of the system of Fig. 3 isigenerally similar to that of Fig. 1 described above.

Operation of the push-button switch 63 is effective to unbalance the network comprising the tube 28 in the opposite sense and cause the path 282) to assume the network current. This operation is effective to develop between the terminals ll) and it a control signal which is applied through the reversing switch 42 to the impedance paths 22a, thereby to energize one of the field windings 60b of the motor 68. The motor 6B thereupon rotates in a predetermined direction to adjust the tuning apparatus of the receiver, as in the system of Fig. 1. Simultaneously the closing of the contacts 63b of the switch 63 is effective to develop a negative potential across the condenser lit in the A. V. C. connection, thereby temporarily to reduce the gain of the receiver so that there is no tendency of any control signal developed by the discriminator M to gain control of the motor 60 and retune it to the station which was previously being received. The time constants of the resistor 65 and condenser 6t and the associated A. V. C. circuit are so selected that this negative potential upon the condenser 58 substantially leaks off within the period required to tune the receiver from the station being received by an amount beyond the range of control of the discriminator l4.

As indicated in the discriminator characteristic of Fig. 2, as the receiver is tuned approximately to a station, the control signal developed thereby decreases from a maximum to Zero at the center of the channel and then increases with opposite polarity as the receiver is tuned through the station. Since the characteristic of the motor Bil is to increase its speed with decreased field excitation, it is apparent that, with this arrangement alone, the motor would tend to increase in speed as it approached the center of the channel to which the receiver was being tuned, while it is preferable to approach such a position at progressively decreasing speed. To avoid this characteristic, the time-constant circuits 61, 68 and ill are included in series in the controlgrid circuits of the impedance paths 22a, 221), respectively. These circuits are eiiectively differentiating circuits which are effective to advance the signal on the control grids so as to anticipate the approach to the center of the channel to which the receiver is being tuned. The signal thus differentiated reverses polarity before the receiver is tuned to the center of the channel so that the motor to coasts into the center of the channel under reverse field excitation, providing a dynamic breaking of the motor. If the motor should stop before it coasted to the center of the channel, the result would be so to modify the differentiated control signal of the discriminator M as again to reverse its polarity and cause the motor to rotate in the original direction at a greatly reduced speed; that is, the motor effectively crawls toward the center of the channel to which the receiver is being tuned.

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed as new is:

1. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, power means for actuating said tuning-apparatus, an electrical net- 2. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, electric motor means I01 actuating said tuning apparatus, adjustable vacuum-tube impedance means for controlling the energization oi said motor means, an electrical network ior controlling said impedance means including parallel paths having (1111616115 impedances so that said network is normally unbalanced, manually operabie means ior unbalancing said network in an opposite sense to initiate operation or said electric motor means, and means responsive to the reception or a signal by the receiver 101 restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus.

an automatic signal-seeking tuner for a radio receiver comprising, an adJustable tuning apparatus for the receiver, reversible electric motor means 101 actuating said tuning apparatus,

vacuum-tube means including 2, P311. 01 aGJuSfiabie-impeoance space-current patns connected selectively to control the energization oi said motor means for operation in either direction, means including a normally unbalanced electrical network ior controlling the impedance of one or the other of said paths, manually operable means Ior selectively unbalancing said network in an opposite sense to initiate operation of said motor means in either direction and means responsive to the reception of a signal by the receiver ror restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus.

4. An automatic signal-seeking tuner for a radio receiver comprising, an ad ustabie tuning apparatus for the receiver, reversible electric motor means Ior actuating said tuning apparatus, vacuum-tube means including a pair of almostable-impedance space-current paths connected selectively to control the energization or said motor means ior operation in either direction, means including a normally-unbalanced electrical network ror controlling the impedance or one or the other of said paths, manually operable means for selectively unbalancing said network in an opposite sense to initiate operation of said motor means in either direction, means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus, and means responsive to adjustment or said tuning apparatus to either of its extreme positions for reversing the polarity of the output of said network to reverse the direction of operation or said motor means.

5. An automatic signal-seeking-tuner for a radio receiver comprising, an ad ustable tuning apparatus for the receiver, power means ior actuating said tuning apparatus, a normally-unbalanced electrical network for controlling said power means, manually operable means for unbalancing said network in an opposite sense to initiate operation of said power means,-means responsive to the reception of a signal by the receiver for restoring said network to its initial.

sense or unbalance to arrest actuation of said tuning apparatirs, and additional means re-- sponsi've to the reception of a signal for apply ing an auxiliary control signal to said power means until the receiver is accurately tuned to the received signal.

6. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, power means for actuating said tuning apparatus, a normallyunbalanced electrical network for controlling said power means. manually operable means for unbalancing said network in an opposite sense to initiate operation of said power means, means responsive to the reception of a'signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus, and a frequency discriminator for applying an auxiliary control signal to said power means until the receiver is accurately tuned to the received signal.

7. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver. power means for actuating said tuning apparatus, a normally-unbalanced electrical network for developing a control signal, a frequency-discriminator network for developing an auxiliary control signal, means responsive jointly to said control signals for controlling said power means, manually operable means for unbalancing said network in an opposite sense to initiate operation of said power means. and means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus when said auxiliary control signal is reduced to zero.

8. An automatic signal-seeking tuner for a radio receiver comprising, an adiustable tuning apparatus for the receiver, power'means for actuating said tuning apparatus, a normally-unbalanced electrical network for developing a control signal, a frequency-discriminator network for develo ing an auxiliary control signal of opposite polarities for mistuning of the receiver in opposite senses. means responsive jointly to the ma nitudes and polarities of said two signals for controlling said power means, manually operable means for unbalancing said network in an opposite sense to initiate operation of said power means and means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus when said auxiliary control signal is reduced'to zero.

9. An automatic si nal-seeking tuner for a radio receiver comprising, an adiustable tuning apparatus for the receiver, power means for actuating said tuning apparatus an electrical network including a pair of space-current paths in parallel and couplings therebetween efifective to cause either path rendered conductive to assume all of the current of the network and an output circuit effective to develop a control signal for controllin said power means. the impedance of one path being greater than that of the other so that said other path is normally conductive manually operable means for reducing the impedance of said one path so that said one path is rendered conductive to modify said control signal, thereby to initiate operation of said power means, and means responsive to the reception of a signal by the receive'rfor restoring said network to its initial condition to arrest actuation of Said tuning apparatus.

1a 10. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, power means for actuating said tuning apparatus, a normally unbalanced electrical network for controlling said power means including a pair of space-current paths in parallel and couplings therebetween effective to cause either path rendered conductive to assume all of the current of the network and to cause a given one of said paths initially to assume such network current upon energization of the network and an output circuit efiective to develop a control signal only when the other of said paths assumes the network current, manually operable means for unbalancing said network to cause said other path to assume the network current, thereby to initiate operation of said power means, and means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus.

11. An automatic signal-seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, power means for actuating said tuning apparatus, a normallyunbalanc'ed electricalnetwork for controlling said power means including a pair of space-current paths in parallel and couplings therebetween effective to cause either path rendered conductive to assume all of the current of the network and to cause a given one of said paths initially to asslime such network current upon energization of the network and an output circuit effective to develop a control signal only when the other of said paths assumes the network current, manually operable means for unbalancing said network to cause said other path to assume the network current, thereby to initiate operation of said power means, and means for applying a received signal to said other of said paths to render it non-conductive, thereby to restore said network to its initial sense of unbalance to arrest actuaticn of said tuning apparatus.

12. An automatic signal-seeking tuner for a radio receiver including an adjustable gain control comprising, an adjustable tuning apparatus for the receiver, power means for actuating said tuning apparatus, an electrical network for controlling said power means including parallel paths having different impedances so that said network is normally unbalanced, manually operable means for unbalancing said network in an opposite sense to initiate operation of said power means, and 'for temporarily reducing the gain of the receiver to reduce the effect of any signal then being received, and means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus.

13. An automatic signal seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, a reversible electric motor for actuating said ap aratus including a winding adapted to be selectively energized in opposite polarities for causing operation of said motor in opposite directions, a pair of transformers connected to said winding for controlling the current therein, vacuum tube means including a pair of adjustable-impedance space-current paths connected individually to said transformers to control the polaritv of the current supplied to said winding, means including a normally unbalanced electrical network for controlling the impedance of one or the other of said paths, manually operable means for selectively unbalancing said network in an opposite sense to initiate operation of said motor means in either direction and means responsive to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuaton of said tuning apparatus.

14. An automatic signal seeking tuner for a radio receiver comprising, an adjustable tuning apparatus for the receiver, a reversible direct current motor for actuating said apparatus including a pair of field windings adapted to be selectively excited for causing operation of said motor in opposite directions, vacuum tube means including a pair of adjustable-impedance spacecurrent paths connected individually to said windings to control operation of said motor, means including a normally unbalanced electrical network for controlling the impedance of one or the other of said paths, manually operable means for selectively unbalancing said network in an opposite sense to initiate operation of said motor means in either direction and means re sponse to the reception of a signal by the receiver for restoring said network to its initial sense of unbalance to arrest actuation of said tuning apparatus.

ELMER G. HILLS.

REFERENCES CITED The following references are of record in the iile of this patent:

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